Method and means for controlling electromagnetic wave energy leakage in microwave ovens

ABSTRACT

A method and means for controlling electromagnetic wave energy leakage in microwave ovens. The hinges, latches, and other pairs of metallic protruding members that are located in proximity of the perimeter of the door of the microwave oven are separated by distance equal to a multiple of 1/2 wave lengths of the fundamental operating frequency of the magnetron. A method for preventing the escape of an excessive amount of electromagnetic wave energy from around the perimeter of the microwave oven door including the steps of monitoring the field of electromagnetic wave energy in the vicinity of the perimeter of the microwave oven door and mounting a pair of metallic reflective projections across the path of any excessive leakage of electromagnetic wave energy.

United States Patent 1 Jarvis 1 May 29, 1973 [54] METHOD AND MEANS FOR 3Primary ExaminerJ. V. Truhe CONTROLLING ELECTROMAGNETIC AssistantExaminerHugh D. Jaeger w v ENERGY LEAKAGE N Attorney-Franklin D.Jankosky, Walter R. Thiel, MICROWAVE OVENS Alfred B. Levine [75]Inventor: George A. Jarvis, Minneapolis, [57] ABSTRACT Mmn. AssigneezLitton y Inc, Beverly Hills, A method and means foncontrollmgelectromagnetic Califwave energy leakage n microwave ovens. The hinges,

- latches, and other pairs of metallic protruding mem- [22] Filed: Nov.15, 1971 bers that are located in proximity of the perimeter of the doorof the microwave oven are separated by [21] Appl' 198802 distance equalto a multiple of k wave lengths of the fundamental operating frequencyof the magnetron. A [52] US. Cl ..219/l0.55 method for preventing theescape of an excessive lnt. amount of electromagnetic wave energy fromaround of Search the perimeter of the microwave oven door including tthe steps of monitoring the field of electromagnetic References cuedwave energy in the vicinity of the perimeter of the UNITED STATESPATENTS microwave oven door and mounting a pair of metallic reflectiveprojections across the path of any excessive 3,448,232 6/1969 Kluck t..2l9/l0.55 leakage of electromagnetic wave energy. 3,633,564 1/1972Togashi ..2l9/10.55 X 3,629,537 12/1971 Haagensen ..219/10.55 2 Claims,3 Drawing Figures Patented Ma 29, 1973 3,736,398

B E W GEORGE A. JARV/S 27 INVENTOR wmjg lwusghu ATTORNEY FIELD OF THEINVENTION This invention relates to electromagnetic wave energy sealsfor microwave ovens and more particularly to auxiliary means and amethod for preventing the escape of electromagnetic wave energy fromsuch ovens.

DESCRIPTION OF THE PRIOR ART The typical microwave oven includes acavity wherein foodstuffs may be placed to be cooked by exposing thefoodstuffs to electromagnetic wave energy radiated by a magnetron. As apractical matter, access must be provided to facilitate the insertionand the removal of the foodstuffs to and from the cavity of themicrowave oven. Pivotally supported doors have been generally utilizedto provide such access. As is wellknown, electromagnetic wave energy inthe vicinity of the perimeter of the door will escape from the cavity ofthe microwave oven unless proper measures are taken to block such escapeof electromagnetic wave energy.

In the past, microwave oven manufactures have utilized various types ofelectromagnetic wave energy seals to prevent the escape ofelectromagnetic wave energy from the cavity of the microwave oven.Originally, microwave ovens featured direct metallic contact between theinside surface of a metallic door and the front surface of a metallichousing, with the addition of spring biasing of the door to maintaincontact between the two surfaces. However, the spring biasing method didnot prove satisfactory over an extended period of use because of warpingof the metallic surfaces or because of deterioration of the springs. Ineither case, arcing between the respective surfaces of the door and thehousing and subsequent excessive leakage of electromagnetic wave energyoccurred unless proper maintenance produres were strictly observed.

Subsequent approaches included the insertion of a number of non-metallicspacers or a conductive rubber gasket between the door and the microwaveoven housing to establish a gap so as to prevent arcing of theelectromagnetic wave energy. Usually these approaches included arecessed area having a terminating conducting surface located anintegral number of k wave lengths of the fundamental frequency of themagnetron from the origin of the gap, or included a series of recessedareas having a depth equal to approximately wave length of thefundamental frequency of the magnetron. In some instances, such recessedareas have been made more compact and have been filled with a dielectricmaterial to maintain the same electrical wave length.

While these prior electromagnetic wave energy seals were originallyacceptable, the recently adopted standards of the Department of Health,Education and Welfare requiring all microwave ovens to have a radiationleakage level of less than I mw/cm upon sale and being capable ofmaintaining a radiation leakage level of less than 5 mw/cm under alloperating conditions preclude many such electromagnetic wave energyseals from being utilized in microwave ovens sold in the United States.It is well-known that household appliances are subjected to abuse, whichin the case of microwave ovens, could impair the effectiveness of thedoor seal. The cost of replacing the door entirely, or even of the doorseal, can be quite expensive.

Accordingly, it is the object of the present invention to provideauxiliary means to cooperate with the original door seal to prevent theescape of electromagnetic wave energy from the cooking cavity of themicrowave oven.

It is a further object of the present invention to provide a door for amicrowave oven wherein any pair of projections located either on theinner surface of the door, or the surface defining the access apertureof the cavity of the microwave oven, are spaced in accordance with theoperating frequency of the magnetron of the microwave oven to preventthe escape of electromagnetic wave energy from the cooking cavity of themicrowave oven.

It is a still further object of the present invention to provide themicrowave oven Serviceman with a method for correcting any excessiveleakage of electromagnetic wave energy from the cooking cavity of themicrowave oven.

It is another object of the present invention to provide stick-onmetallic strips which may be readily placed on the interior surface ofthe microwave oven door, or on the surface defining the access apertureof the cooking cavity, to prevent the escape of electromagnetic waveenergy from the cooking cavity of the microwave oven.

SUMMARY OF THE INVENTION In accordance with the objects set forth above,the present invention provides auxiliary means for preventing the escapeof electromagnetic wave energy from the cooking cavity of a microwaveoven. In the construction of the microwave oven, any pair of metallicprotruding members, located in proximity of the perimeter of the door ofthe microwave oven, for example, the hinges or the latching members, areseparated by a multiple of A wave lengths of the fundamental operatingfrequency. If excessive leakage of electromagnetic wave energy from thecooking cavity occurs after use of the microwave oven, a method ofattenuating such leakage includes the placing of a pair of metallicreflective projections, spaced at a multiple of A wave lengths of thefundamental operating frequency, either on the interior surfaces of themicrowave oven door or on the surface defining the access aperture ofthe cooking cavity.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects, advantages andcharacteristic features of the present invention will become readilyapparent from the following detailed description of the preferredembodiments of the invention when taken in conjunction with thedrawings, in which:

FIG. 1 is a perspective view of a microwave oven in accordance with thepresent invention;

FIG. 2 is an elevational view of the interior side of the microwave ovendoor in accordance with the present invention; and

FIG. 3 is a side elevational view of the microwave oven door taken alongline 3-3 of FIG. 2 in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a perspective view of a microwave oven 10, in accordance withthe principles of the present invention. The exterior of the microwaveoven door is generally comprised of a housing 11, a door 12 and acontrol panel 13. The control panel 13 includes a conventional timer andan indicator light. The door 12 includes a window 14, or viewing area,which consists of an RF screen sandwiched between two panes of glasswhich enables one to view the cooking of the foodstuffs which are placedwithin the cooking cavity 15 of the microwave oven 10.

The door 12 is shown in its open position so that features of thepresent invention may be more clearly illustrated. The door 12 ismovably supported by a pair of hinges 16a and 16b which are partiallyshown in FIG. 1, and are more clearly shown in FIGS. 2 and 3. The door12 also includes a handle 17 and a pair of latching members 18a and 18bwhich, upon closure of the door 12, engage a pair of concealedcooperating latching and interlock members located behind the openings19a and 19b of the front panel 20. Further illustrated is movable member21, one end of which is mounted within the door 12 via opening 21a, theother end of the movable member 21 cooperates with a concealed door stopwithin the microwave oven 10 to limit the swing of the door 12.

Referring now to FIGS. 2 and 3, there is respectively shown anelevational view of the interior side of the door 12 and sideelevational view of the door 12 in accordance with the principles of thepresent invention. The interior side of the door 12 is principallycomprised of an interior surface panel 22, a choke 23, a flexible sealplate 24 and the window 14. The choke 23 and the flexible seal plate 24cooperate to form an effective seal to prevent the escape ofelectromagnetic wave energy from the cooking cavity 15. The choke 23 iscomprised of a continuous piece of suitable dielectric material, such aspolypropyleyne, which is located within a recessed metallic area of theinterior surface panel 22. The primary purpose of the choke 23 is toprevent the escape of the fundamental operating frequency of 2,450 Ml-Izfrom the cooking cavity 15. The flexible seal plate 24 abuts the frontpanel when the door 12 is closed to form a capacitive seal to preventthe escape of electromagnetic wave energy of harmonics of thefundamental operating frequency from the cooking cavity 15.

During the development of the microwave oven 10, it was discovered thatif a pair of metallic reflective projections separated by some multipleof wave length of the fundamentalfrequency of 2,450 MHz, were placed oneither the front panel 20 or-on the interior surface panel 22 in thepath of escaping electromagnetic wave energy, the level of such leakagewas decreased. The door 12 was then held in its closed posi tion withoutthe aid of any hinges or latches, and an acceptable level of leakage ofelectromagnetic wave energy existed. When the necessary respective pairsof hinges and latches were placed at respective random locations on thedoor 12, the leakage of electromagnetic wave energy increased. However,if the respective pair of hinges and latches were separated by somemultiple of k wave length of the fundamental operating frequency of2,450 MHz, the leakage of electromagnetic wave energy was decreased.Thus, it was determined that strategic placement of any pair of metallicreflective projections would serve as additional means to prevent theescape of electromagnetic wave energy from the cooking cavity 15. Thepair of hinges 16a and I6 b are separated by the distance A, as shown,and

the pair of latching members 18a and 18b are separated by the distanceB, as shown. The respective lengths of A" and 8" are multiples of A wavelengths of the fundamental operating frequency of 2,450 MHz, morespecifically, the distances A and B are 2 lambda and l-k lambda of 2,450MHz.

Referring now to FIGS. 1, 2 and 3, a plurality of pairs of metallicreflective projections are illustrated which may be placed either on theinterior surface panel 22 or on the front panel 20 to attenuate theescape of electromagnetic wave energy from the cooking cavity 15. As iswell-known, if a microwave oven is subjected to abuse, the door sealwill not effectively attenuate electromagnetic wave energy attempting toescape from the cooking cavity via any gap existing around the perimeterof the door of the microwave oven. During an exhaustive test conductedon the microwave oven illustrated in FIG. 1, wherein the door 12 wasopened and closed a great many times, it was also found that the escapeof electromagnetic wave energy would increase on random points aroundthe perimeter of the door 12. Pairs of metallic reflective projectionswere strategically placed, across such points of escape, either on interior surface panel 22 or the front panel 20 at a distance apart equalto, or approximately equal to, k wave lengths of the operatingfrequency, and escape of electromagnetic wave energy from the cookingcavity 15 was attenuated.

Depending upon the gap set up between the interior surface panel 22 andthe front panel 20, the metallic reflective projections varied inthickness between 0.01 inches and 0.05 inches. In FIG. 1, metallicreflective projections 25a and 25b are located on the front panel 20 ata distance C which is multiple 9% wave lengths of operating frequency.In FIGS. 2 and 3, the respective pairs of metallic reflectiveprojections 26a and 26b, and 27a and 27b are respectively located on theinterior surface panel 22 at distances D" and E, which are approximatemultiples of 1% wave lengths of the fundamental operating frequencies.

In the past, microwave oven servicemen would periodically check for anyexcessive leakage of electromagnetic wave energy from the perimeter ofthedoors from microwave ovens that were in the field. In order to obtainan accurate reading of the escape of such electromagnetic wave energy, apower density meter is placed within 5 cm. of the perimeter of the door12 and the power density meter is moved around the entire perim eter ofthe door 12. In the case of the method ofproviding auxiliary means toprevent the escape of electromagnetic wave energy from around theperimeter of the door, as practiced by the present invention, aserviceman may monitor the field of electromagnetic wave energy existingaround the perimeter of the door 12. If the field of electromagneticwave energy is approximately 5 mw/cm or higher at any point around theperimeter of the door, the serviceman may then mount a pair of metallicreflective projections on either the interior surface panel 22 or on thefront panel 20, as illustrated in FIGS. 1, 2 and 3 in the path of suchescape of electromagnetic wave energy. The respective electromagneticwave energy in such paths then encounters the pair of metallicreflective projections and a current is established between the twometallic reflective projections. As the electromagnetic wave energy isreflected back and forth between the two metallic reflectiveprojections, the electromagnetic wave energy is attenuated to a levelsafely below 5 mw/cm In some cases, it was found when theelectromagnetic wave energy in one point was attenuated by the use of apair of metallic reflective projections, there was an increase ofelectromagnetic wave energy at some other point along the perimeter ofthe door, though such increased level of leakage of electromagnetic waveenergy at the other was not necessarily above the minimum safe level.Thus, in effect, the serviceman is sometimes balancing the leakage ofelectromagnetic wave energy around the perimeter of the door 12 so thatthe leakage level is not above 5 mw/cm at any single point.

The serviceman may be provided with metallic reflective projectionswhich may be mounted on the interior surface panel 22 or on the frontpanel by means of a conventional adhesive, or the serviceman may beprovided with metallic reflective means which are comprised of metallicstrips coated on one side with an adhesive that is protected by aremovable cover. The serviceman may monitor the microwave oven todiscover any point of excessive leakage, and upon such discovery, he mayreadily tear off the protective covers of a pair of metallic reflectivemeans and properly mount them on the microwave oven as describedearlier.

It should be understood that the present invention is not limited topairs of hinges, latches or mounted metallic reflective projectionsseparated by a distance equal to some multiple of approximately 1% wavelength of the fundamental operating frequency. For example, in somecases, because of the physical dimensions of the cooking cavity, orbecause of the gap set up between the interior surface panel of the doorand the front panel of the microwave housing, it may be necessary tobuild a microwave oven wherein pairs of embossed projections arefabricated as part of the metallic interior surface panel or themetallic front housing panel to provide an effective seal against theleakage of electromagnetic wave energy. In addition, if the microwaveoven utilizes only one latch, a metallic reflective projection may bepaired with it to provide an effective auxiliary seal in accordance withthe principles of the present invention.

Thus, although the present invention has been shown and described withreference to particular embodiments, for example, pairs of metallicreflective projections which may be mounted apart at a distance equal tomultiples of h wave lengths of the fundamental operating frequency,nevertheless, various changes and modifications obvious to personsskilled in the art to which this invention pertains, for example, theuse of three metallic reflective projections spaced to provide twoseparate multiples of h wave lengths of the fundamental operatingfrequency, are deemed to lie within the spirit, scope and contemplationof the invention as set forth in the appended claims.

What is claimed is:

l. A method for controlling electromagnetic wave energy leakage betweena door and a front face ofa microwave oven comprised of the steps of:

measuring the magnitude of electromagnetic wave energy leakage existingin proximity of said door of said microwave oven; and placing a pair ofmetallic reflective members on said front surface to form a hollowrectangular slot within the path of said electromagnetic wave energyleakage at a distance apart that is approximately equal to a multiple of/2 wavelengths of the fundamental frequency of the electromagnetic waveenergy generated within said microwave oven.

2. A method for controlling electromagnetic wave energy leakage betweena door and a front face of a microwave oven as recited in claim 1 whichfurther includes the step of:

removing a pair of adhesive coated metallic reflective members from aprotective cover prior to placing said pair of metallic reflectivemembers on said front surface of the microwave oven.

1. A method for controlling electromagnetic wave energy leakage betweena door and a front face of a microwave oven comprised of the steps of:measuring the magnitude of electromagnetic wave energy leakage existingin proximity of said door of said microwave oven; and placing a pair ofmetallic reflective members on said front surface to form a hollowrectangular slot within the path of said electromagnetic wave energyleakage at a distance apart that is approximately equal to a multiple of1/2 wavelengths of the fundamental frequency of the electromagnetic waveenergy generated within said microwave oven.
 2. A method for controllingelectromagnetic wave energy leakage between a door and a front face of amicrowave oven as recited in claim 1 which further includes the step of:removing a pair of adhesive coated metallic reflective members from aprotective cover prior to placing said pair of metallic reflectivemembers on said front surface of the microwave oven.